[gn] Make v8_target_arch a GN declare_arg()

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-# Because standalone V8 builds are not supported, assume this is part of a
-# Chromium build.
+import("gni/v8.gni")
 import("//build_overrides/v8.gni")
 
 import("snapshot_toolchain.gni")
 
 declare_args() {
   # Enable the snapshot feature, for fast context creation.
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
   v8_use_snapshot = true
 
   # Use external files for startup data blobs:
@@ skipping 37 matching lines @@
   # Similar to vfp but on MIPS.
   v8_can_use_fpu_instructions = true
 
   # Similar to the ARM hard float ABI but on MIPS.
   v8_use_mips_abi_hardfloat = true
 }
 
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
-if (is_msan) {
-  # Running the V8-generated code on an ARM simulator is a powerful hack that
-  # allows the tool to see the memory accesses from JITted code. Without this
-  # flag, JS code causes false positive reports from MSan.
-  v8_target_arch = "arm64"
-} else {
-  v8_target_arch = target_cpu
+if (v8_target_arch == "") {
+  if (is_msan) {
+    # Running the V8-generated code on an ARM simulator is a powerful hack that
+    # allows the tool to see the memory accesses from JITted code. Without this
+    # flag, JS code causes false positive reports from MSan.
+    v8_target_arch = "arm64"
+  } else {
+    v8_target_arch = target_cpu
+  }
 }
 
 if (v8_use_snapshot && v8_use_external_startup_data) {
   snapshot_target = ":v8_external_snapshot"
 } else if (v8_use_snapshot) {
   snapshot_target = ":v8_snapshot"
 } else {
   assert(!v8_use_external_startup_data)
   snapshot_target = ":v8_nosnapshot"
 }
@@ skipping 2100 matching lines @@
   sources = [
     "test/fuzzer/wasm-asmjs.cc",
   ]
 
   deps = [
     ":fuzzer_support",
   ]
 
   configs = [ ":internal_config" ]
 }